When transmitting electromagnetic microwaves through hollow waveguides, it is often desirable to be able to twist the direction of the electrical field vector. One arrangement for effecting such rotation is known, for instance, from UK Patent Specification No. 1 299 032. The arrangement includes a number of segments each having a length of one-quarter of a wave length and the segments being displaced angularly in relation to one another. The segments all have substantially the same cross-sectional shape and the device is intended to be connected at both ends thereof to hollow waveguides of rectangular cross-section. The U.S. Pat. No. 4,620,163 teaches devices for rotating the electrical field vector of a microwave. The devices are rectangular in shape at both ends thereof and merge continuously between the crosssections of said ends. It is known to use so-called ridge waveguides in many applications of hollow waveguides, for instance for radar antennas. Ridge waveguides are substantially rectangular in cross-section, but deviate from a true rectangular shape by virtue of a ridge which projects into the waveguide from one side thereof. Ridge waveguides are found described, for instance, in the second edition of "Introduction to Microwaves" by Fred E. Gardiol, Artech House, 1984. When using ridge waveguides for the aforesaid purposes, it is often desirable to be able to supply power to said waveguides via rectangular, hollow waveguides and to twist the electrical field vector as power is being supplied. Devices for effecting such power supply are known in which the field vector in the rectangular waveguide is first twisted through one-quarter of a revolution, whereafter the rectangular waveguide changes to a ridge waveguide. These devices are complicated and relatively bulky and are less suitable for use, e.g. in mobile radar antennas.